wrx rough idle

I have noticed on every wrx Ive driven that idle with the A/C onis a shakey affair.When the fans that cool the engine or the A/C is off its quite smooth.My legacy outback 2.5 is very smooth but my imprezza 2.2 I had in 1998 hadthe same rocky idle when any load is put on the engine.Can anyone tell me thats done the grounding mod if that smooths the idle out. Ive read a few posts that indicate that.